Distortion correcting sensors for diagonal collection of oblique imagery

ABSTRACT

A vehicle collects oblique imagery along a nominal heading using rotated camera-groups with optional distortion correcting electronic image sensors that align projected pixel columns or rows with a pre-determined direction on the ground, thereby improving collection quality, efficiency, and/or cost. In a first aspect, the camera-groups are rotated diagonal to the nominal heading. In a second aspect, the distortion correcting electronic image sensors align projected pixel columns or rows with a pre-determined direction on the ground. In a third aspect, the distortion correcting electronic image sensors are rotated around the optical axis of the camera. In a fourth aspect, cameras collect images in strips and the strips from different cameras overlap, providing large-baseline, small-time difference stereopsis.

CROSS REFERENCE TO RELATED APPLICATIONS

Benefit claims for this application are made in the accompanyingApplication Data Sheet. This application incorporates by reference forall purposes the following applications, all commonly owned with theinstant application:

-   -   U.S. Non-Provisional application (Docket No. TL-12-01US-B) Ser.        No. 14/774,918, filed Sep. 11, 2015, now U.S. Pat. No.        9,503,639, first named inventor Iain Richard Tyrone MCCLATCHIE,        and entitled DISTORTION CORRECTING SENSORS FOR DIAGONAL        COLLECTION OF OBLIQUE IMAGERY;    -   PCT Application Serial No. (Docket No. TL-12-01PCT) Serial No.        PCT/US2014/030058, filed Mar. 15, 2014, first named inventor        Iain Richard Tyrone MCCLATCHIE, and entitled DISTORTION        CORRECTING SENSORS FOR DIAGONAL COLLECTION OF OBLIQUE IMAGERY;        and    -   U.S. Provisional Application (Docket No. TL-12-01) Ser. No.        61/786,311, filed Mar. 15, 2013, first named inventor Iain        Richard Tyrone MCCLATCHIE, and entitled DIAGONAL COLLECTION OF        OBLIQUE IMAGERY.

BACKGROUND Field

Advancements in photogrammetry are needed to provide improvements inperformance, efficiency, and utility of use.

Related Art

Unless expressly identified as being publicly or well known, mentionherein of techniques and concepts, including for context, definitions,or comparison purposes, should not be construed as an admission thatsuch techniques and concepts are previously publicly known or otherwisepart of the prior art. All references cited herein (if any), includingpatents, patent applications, and publications, are hereby incorporatedby reference in their entireties, whether specifically incorporated ornot, for all purposes.

An example of a camera is an image capturing system that capturesimagery using a lens that focuses light on at least one Petzval surface(e.g., a focal plane), and captures an image with at least one imagesensor on the Petzval surface. A focal plane is an example of a planarPetzval surface. In some scenarios, Petzval surfaces are not necessarilyplanar and are optionally curved due to the design of the lens. Examplesof image sensors include film and electronic image sensors. Examples ofelectronic image sensors include Charge Coupled Device (CCD) sensors andComplementary Metal-Oxide Semiconductor (CMOS) sensors, such as thosemanufactured by Aptina. An example of an emerging optical axis of acamera is the path along which light travels from the ground at thecenter of the lens field of view to arrive at the entrance to thecamera. The light path inside the camera may be folded with reflectingsurfaces, but eventually light arriving along the emerging optical axiswill converge at the center of the Petzval surface(s). An example of anacute angle is an angle greater than zero degrees and less than 90degrees. An example of an oblique angle, such as an acute or an obtuseangle, is an angle that is not a right angle (e.g. 90 degrees) and isnot a multiple of a right angle (e.g. not modulo 90 degrees).

Some maps assume a camera perspective looking straight down, called anorthographic (or nadir) perspective. In some embodiments and/orscenarios, this is also the perspective of the captured images used tomake these maps (e.g., orthographic imagery). However, orthographicimagery eliminates all information about the relative heights ofobjects, and information about some surfaces (e.g., the vertical face ofa building).

Other maps assume a camera perspective looking down at an angle belowthe horizon but not straight down, called an oblique perspective. Anexample of a down angle of a camera is the angle of the emerging opticalaxis of the camera above or below the horizon; down angles for nadirperspectives are thus 90 degrees; down angles for oblique perspectivesare e.g., 20 to 70 degrees. In some embodiments and/or scenarios, thecamera used to capture an oblique perspective is referred to as anoblique camera and the resulting images are referred to as obliqueimagery. In some scenarios, oblique imagery is beneficial because itpresents information that is useful to easily recognize objects and/orlocations (e.g., height and vertical surfaces); information that issometimes missing from orthographic imagery.

In some embodiments, the same point on the ground is captured withoblique images captured from multiple perspectives (e.g., 4 perspectiveslooking at a building, one from each cardinal direction: North, South,East, and West). This is sometimes described as ground-centriccollection, and yields ground-centric oblique imagery. In variousscenarios, ground-centric aerial oblique imagery is useful, e.g., forassessing the value of or damage to property, particularly over largegeographic areas. In some scenarios, it is a priority in aground-centric collection program to collect an image of every point insome defined target area for each of the cardinal directions. Thecapture resolution is measured in distance units on the ground (e.g., 4inch per pixel) and sometimes does not vary much between differentpoints in the target area.

In some embodiments, multiple oblique images are captured from a singlepoint, with multiple perspectives (e.g., 4 perspectives looking from abuilding in each cardinal direction), also known as sky-centriccollection. In some scenarios, sky-centric imagery is used to form apanoramic view from a single point. In some scenarios, it is a priorityin a sky-centric collection program to collect a continuous panoramafrom each viewpoint. Capture resolution is sometimes measured in angularunits at the viewpoint (e.g., 20,000 pixels across a 360 degreepanorama).

In various embodiments, a camera-group is a system of one or morecameras that approximately capture the same image (e.g., the opticalaxes are aligned within 5 degrees of a common reference axis). Forexample, an ordinary pair of human eyes acts as a 2 camera-group,focusing on a single image. In various scenarios, a camera-group has anarbitrary number of cameras.

In some embodiments, a camera-set is a system of one or more camerasand/or camera-groups that capture different images. One example of a 2camera-set is a nadir camera and an oblique camera. Another example of a4 camera-set is 4 oblique cameras, each pointing in a different cardinaldirection. In various scenarios, a camera-set has an arbitrary number ofcameras and/or camera-groups.

An example of the nominal heading of a vehicle is the overall directionof travel of the vehicle. In many scenarios, the instantaneous directionof travel deviates from the nominal heading. For example, an airplane isflying along a flight path heading due north, so that the nominalheading is north, while experiencing a wind blowing from west to east.To keep the plane on the flight path, the pilot will point the planeinto the wind, so that the instantaneous heading is many degrees west ofnorth. As another example, a car is driving down a straight road thatruns from south to north and has several lanes. The nominal heading isnorth. However, to avoid hitting an obstacle, the car changes lanes,instantaneously moving northwest, rather than strictly north. Despitethis instantaneous adjustment, the nominal heading is still north. Incontrast, when the car turns 90 degrees from north to travel west, thenominal heading is now west.

An example of a plan angle of an oblique camera on a vehicle is theangle between the nominal heading of the vehicle and the emergingoptical axis of the camera projected onto the ground plane; plan anglesvary from 0-360 degrees. Some cameras are mounted on stabilizationplatforms so that the camera maintains its plan angle even as theinstantaneous heading changes. Some cameras are mounted directly to thevehicle. Note that a vehicle may have a nominal heading, even whenstopped, e.g., a helicopter with a flight path due north could stopperiodically, but would still have a nominal heading of due north.

Camera-sets used for sky-centric collection expend far more film (andlater pixels) on ground points that the vehicle travels directly over,compared to ground points off to the side of the vehicle's path. Whenaerial photography and photogrammetry began to use airplanes, it becameimportant to use less film to reduce costs. Some camera-sets removed theforward- and rear-facing oblique cameras of the earlier designs, andused a nadir camera and two oblique cameras pointing to the side (e.g.,all emerging optical axes approximately perpendicular to the nominalheading of the airplane). While flying in a straight line and capturingoverlapping images, these camera-sets capture the same amount of groundarea with the same resolution as the more complex panoramic camerasand/or camera-sets, but with less film.

The extent of coverage in the direction of flight (sometimes describedas in track) is, in some scenarios, primarily determined by the distanceof flight. The extent of coverage orthogonal to the direction of flight(sometimes described as cross track) is, in some scenarios, primarilydetermined by the plane's altitude and the design of the camera. Theextent of coverage in the cross track direction is sometimes called theswath. One benefit of a camera-set with both an oblique camera and anadir camera is achieving greater swath without complex lens designs(such as a single large Field Of View, e.g., FOV, fisheye).

In some sky-centric collection scenarios, the vehicle is maneuvereduntil the objects of interest are in view. For some ground-centriccollection scenarios, the vehicle moves through a pattern which gives anopportunity to capture each point of interest on the ground from everyrequired direction. In various embodiments, a Maltese Cross camera-setis moved in a path of parallel lines (e.g., flight lines of an airplane)that run in a north-south or east-west direction. As the vehicle movesalong the flight lines, the images captured by any particular camera areoptionally superposed to form a long continuous strip of coverage. Thelength of the strip is approximately the length of the flight line, andthe width of the strip is known as the swath.

FIG. 1 conceptually illustrates an isometric view of selected prior artdetails of an airplane 102 with a Maltese Cross style obliquecamera-set. The sensor fields of view of the forward 104, right 106,back 108, and left 110 oblique cameras are shown, projected onto theground. The emerging optical axes of the cameras (respectively 112, 114,116, and 118) have 45 degree down angles. Down Angle 122 is the angleformed between the Emerging Optical Axis 114 and its projection 120 to aplane parallel to the ground. For clarity, the other down angles areomitted from the illustration.

FIG. 2 conceptually illustrates a plan view of selected prior artdetails of the field of view of a single example camera of a MalteseCross camera-set. The conical field of view projects from cameraaperture 208 to an ellipse 202 on the planar surface, with the longermajor axis of the ellipse pointing away from the center of the camera.The image formed by the lens is a circle 210, shown at the left at alarger scale, and looking down the lens optical axis. The image sensoris an inscribed rectangle 212 that projects to a trapezoid 204 on thesurface, because of the down angle of the camera. The image sensor is arectangular array of pixels arranged in rows 220 and columns 216. Lightrays 206 corresponding to the four corners of the image sensor are alsoshown. The light rays come from the ground up through the lens to thesensor. The pixels of the image sensor are projected onto the ground,forming projected rows 218 and projected columns 214. In the example,the rectangular image sensor is 24 mm by 36 mm, the focal length is 100mm, and the camera altitude above the surface is 1000 meters. Theresulting trapezoid is 455 meters wide at its base and 579 meters wideat its top.

FIG. 3 conceptually illustrates a plan view of selected prior artdetails of capturing oblique imagery via a Maltese Cross camera-set. Invarious embodiments, the nominal heading of vehicle 301 is a cardinaldirection (e.g., North, South, East, West). The camera-set includes fouroblique cameras, with 0, 90, 180, and 270 degree plan angles. Forconceptual clarity, the emerging optical axes are drawn in FIG. 3 with a3 degree offset. Each camera has the same focal length and sensor sizeas the example camera in FIG. 2. However, the left and right camerashave the longer 36 mm dimension of the sensors aligned with the nominalheading. The projected FOV ellipses of the cameras 304, 308, 312, and316 contain the projected sensor FOV trapezoids, respectively 302, 306,310, and 314. Several captured images 320 of the projected FOVtrapezoids are shown. The captured images from a single camera in asingle flight line form a continuous strip, and there is, in somescenarios, relatively significant forward overlap between images in thestrip (e.g., from 50% to 60% overlap between sequentially capturedimages).

The swaths of the front- and rear-facing cameras are also, in somescenarios, relatively significantly smaller than the separation betweenthe swaths of the side-facing cameras. The front-facing camera swath isbetween edges 352 and 354, and as noted is, e.g., 458 meters wide. Theinner edges of the side facing swaths are denoted by edges 362 and 364,and the space between them 365 is, e.g., 1571 meters.

FIG. 4 conceptually illustrates selected prior art details of an exampleflight plan for capturing oblique imagery covering Alexandria County,Virginia, using the Maltese Cross camera-set of FIG. 3. Flight plan 401is arranged in 25 flight lines (e.g., 402) with nominal headings east orwest, separated by 24 turns (e.g., 403) and captures oblique images thatare oriented north, south, east and west. The total flight distance is264 kilometers.

To capture the views offered by the front and rear facing cameras forevery point of interest on the ground, the vehicle's flight lines arecloser together than the swath of the front and rear facing cameras. Inthe flight plan depicted in FIG. 4, the flight line pitch is 340 meters,so that there is 25% horizontal overlap between adjacent strips ofimagery.

SYNOPSIS

The invention may be implemented in numerous ways, including as aprocess, an article of manufacture, an apparatus, a system, acomposition of matter, and a computer readable medium such as a computerreadable storage medium (e.g., media in an optical and/or magnetic massstorage device such as a disk, or an integrated circuit havingnon-volatile storage such as flash storage) or a computer networkwherein program instructions are sent over optical or electroniccommunication links In this specification, these implementations, or anyother form that the invention may take, may be referred to astechniques. The Detailed Description provides an exposition of one ormore embodiments of the invention that enable improvements inperformance, efficiency, and utility of use in the field identifiedabove. The Detailed Description includes an Introduction to facilitatethe more rapid understanding of the remainder of the DetailedDescription. The Introduction includes Example Embodiments of one ormore of systems, methods, articles of manufacture, and computer readablemedia in accordance with the concepts described herein. As is discussedin more detail in the Conclusions, the invention encompasses allpossible modifications and variations within the scope of the issuedclaims.

In some embodiments, the camera designer chooses whether to align eitherthe projected rows or projected columns of the image sensor with thedirection of flight. In some embodiments, the column vector, projectedonto the ground, is aligned near the nominal heading, leaving the rowvector, projected onto the ground, aligned as near as practical to thecross-track direction. An example of a twist angle of an image sensor isthe angle between the image sensor row vector and a vector at thePetzval surface, orthogonal to the optical axis, and parallel to theground plane (sometimes referred to as the horizontal vector).

In one embodiment, the vehicle carries at least four oblique cameras, atleast one pointed approximately in each of the four diagonal directionsfrom the nominal heading of the vehicle (e.g., 45, 135, 225 and 315degree plan angles). In some embodiments, the flight lines of thecollection flight plan are in the intercardinal directions (northeast,northwest, southeast, or southwest).

In some embodiments, one or more oblique cameras are rotated relative tothe nominal heading of a plane (e.g., 45 degree plan angle). The flightlines of the collection flight plan are in the cardinal directions, andin yet other embodiments the flight lines are in arbitrary directions.In some embodiments, the sensors of the oblique cameras are twisted toalign either the projected rows or projected columns of the image sensorwith the direction of flight.

In another embodiment, the vehicle carries at least four oblique cameraswith distortion correcting electronic image sensors. The electronicimage sensors behind each lens have a twist angle such that the columnsor rows of the projected sensor field of view are approximately alignedwith the nominal heading. In some embodiments, the four oblique camerasare positioned in a Maltese Cross configuration (.e.g., plan angles ofapproximately 0, 90, 180, and 270 degrees), while in other embodimentsthe four oblique cameras are positioned diagonally (e.g., 45, 135, 225and 315 degree plan angles).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 conceptually illustrates an isometric view of selected prior artdetails of an airplane with a Maltese Cross style oblique camera-set.

FIG. 2 conceptually illustrates a plan view of selected prior artdetails of the field of view of a single example camera of a MalteseCross camera-set.

FIG. 3 conceptually illustrates a plan view of selected prior artdetails of capturing oblique imagery via a Maltese Cross camera-set.

FIG. 4 conceptually illustrates selected prior art details of an exampleflight plan for capturing oblique imagery covering Alexandria County,Virginia, using the Maltese Cross camera-set of FIG. 3.

FIG. 5 conceptually illustrates a plan view of selected details of anembodiment of capturing oblique imagery via a camera-set with emergingoptical axes rotated in plan.

FIG. 6 conceptually illustrates a plan view of selected details of anembodiment of capturing oblique imagery via a camera-set with rotatedemerging optical axes and distortion correcting sensors.

FIG. 7 conceptually illustrates selected details of an example flightplan for embodiments of capturing oblique imagery covering AlexandriaCounty, Virginia, using the camera-set of FIG. 6.

FIG. 8A conceptually illustrates selected details of the FOV of theforward camera from two adjacent flight lines for a Maltese Crosscamera-set capturing oblique imagery.

FIG. 8B conceptually illustrates selected details of the FOV of theforward-right camera from two adjacent flight lines for an embodiment ofcapturing oblique imagery via a camera-set with rotated emerging opticalaxes and distortion correcting sensors.

FIG. 9 conceptually illustrates a plan view of an embodiment ofcapturing oblique and nadir imagery via a camera-set with rotatedemerging optical axes and distortion correcting sensors, where the nadirand oblique swaths overlap slightly.

FIG. 10 conceptually illustrates a plan view of selected details ofembodiments of a vehicle traveling diagonally.

FIG. 11 conceptually illustrates a plan view of selected details ofembodiments of a vehicle with a rotated oblique camera-set.

FIG. 12A conceptually illustrates selected details of embodiments of anoblique camera with an electronic image sensor that projects to adistorted sensor field of view.

FIG. 12B conceptually illustrates selected details of embodiments of anoblique camera with a non-uniform distortion correcting electronic imagesensor that projects to a corrected sensor field of view.

FIG. 13 conceptually illustrates selected details of embodiments of adiagonal oblique camera with a rotated distortion correcting electronicimage sensor that projects to a partially corrected sensor field ofview.

FIG. 14 conceptually illustrates selected details of embodiments of adiagonal oblique camera with a rotated array of rotated distortioncorrecting electronic image sensors that projects to an array ofpartially corrected sensor fields of view.

FIG. 15 conceptually illustrates selected logical details of embodimentsof a vehicle-based image collection and analysis system.

FIG. 16 illustrates a flow diagram of selected details of an embodimentof image collection and analysis wherein the vehicle is a plane.

FIG. 17 conceptually illustrates selected physical details ofembodiments of a vehicle-based image collection and analysis system.

DETAILED DESCRIPTION

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures illustrating selecteddetails of the invention. The invention is described in connection withthe embodiments. The embodiments herein are understood to be merelyexemplary, the invention is expressly not limited to or by any or all ofthe embodiments herein, and the invention encompasses numerousalternatives, modifications, and equivalents. To avoid monotony in theexposition, a variety of word labels (including but not limited to:first, last, certain, various, further, other, particular, select, some,and notable) may be applied to separate sets of embodiments; as usedherein such labels are expressly not meant to convey quality, or anyform of preference or prejudice, but merely to conveniently distinguishamong the separate sets. The order of some operations of disclosedprocesses is alterable within the scope of the invention. Wherevermultiple embodiments serve to describe variations in process, method,and/or program instruction features, other embodiments are contemplatedthat in accordance with a predetermined or a dynamically determinedcriterion perform static and/or dynamic selection of one of a pluralityof modes of operation corresponding respectively to a plurality of themultiple embodiments. Numerous specific details are set forth in thefollowing description to provide a thorough understanding of theinvention. The details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof the details. For the purpose of clarity, technical material that isknown in the technical fields related to the invention has not beendescribed in detail so that the invention is not unnecessarily obscured.

INTRODUCTION

This introduction is included only to facilitate the more rapidunderstanding of the Detailed Description; the invention is not limitedto the concepts presented in the introduction (including explicitexamples, if any), as the paragraphs of any introduction are necessarilyan abridged view of the entire subject and are not meant to be anexhaustive or restrictive description. For example, the introductionthat follows provides overview information limited by space andorganization to only certain embodiments. There are many otherembodiments, including those to which claims will ultimately be drawn,discussed throughout the balance of the specification.

EXAMPLE EMBODIMENTS

In concluding the introduction to the detailed description, what followsis a collection of example embodiments, including at least someexplicitly enumerated as “ECs” (Example Combinations), providingadditional description of a variety of embodiment types in accordancewith the concepts described herein; these examples are not meant to bemutually exclusive, exhaustive, or restrictive; and the invention is notlimited to these example embodiments but rather encompasses all possiblemodifications and variations within the scope of the issued claims.

EC1) A method comprising:

-   -   operating a vehicle in accordance with a nominal heading, the        operating comprising having one or more respective camera-groups        each enabled to capture oblique imagery via electronic image        sensor technology;    -   configuring each of the respective camera-groups with a        respective pre-determined plan angle range; and    -   establishing the nominal heading as corresponding to a cardinal        direction plus a pre-determined angular offset between 10 and 80        degrees, and capturing oblique imagery in at least one cardinal        direction with at least one camera of the respective        camera-groups.

EC2) The method of EC1, wherein at least one of the respectivecamera-groups comprises a single camera.

EC3) The method of EC1, wherein at least one of the respectivecamera-groups comprises multiple cameras.

EC4) The method of EC1, wherein the respective camera-groups comprise Nparticular camera-groups, each of the N particular camera-groups isassociated with a unique integer K between 0 and (N−1) inclusive, andthe respective pre-determined plan angle range of the particularcamera-group is (180+360*K)/N degrees plus a pre-determined angularoffset range.

EC5) The method of EC4, wherein the pre-determined angular offset rangeis between minus 120/N and plus 120/N degrees.

EC6) The method of EC5, wherein N is four or eight.

EC7) The method of EC1, further comprising configuring a particularelectronic image sensor of a particular one of the respectivecamera-groups in an orientation to reduce angular separation between thenominal heading and one of a projected pixel column and a projectedpixel row of the particular electronic image sensor below apre-determined separation threshold.

EC8) The method of EC7, wherein the pre-determined separation thresholdis 30 degrees.

EC9) The method of EC7, wherein the configuring comprises rotating theparticular electronic image sensor around an optical axis of a camera ofthe particular camera-group.

EC10) The method of EC1, wherein the vehicle further comprises at leastone nadir camera-group enabled to capture nadir imagery.

EC11) The method of EC10, wherein a sensor field of view within thenadir camera-group overlaps a sensor field of view within at least oneof the respective camera-groups.

EC12) The method of EC1, wherein the vehicle is one or more of anaircraft, an airplane, a lighter-than-air craft, a space-craft, ahelicopter, a satellite, a car, a truck, a land-based vehicle, a ship, aboat, a barge, a canoe, a submersible, and a submarine.

EC13) The method of EC12, wherein the vehicle is unmanned or manned

EC14) The method of EC1, wherein at least one electronic image sensor ofthe respective camera-groups is enabled to capture infrared radiation.

EC15) The method of EC1, wherein at least one of the respectivecamera-groups comprises an electronic image sensor.

EC16) The method of EC1, wherein at least one camera of the respectivecamera-groups comprises at least one partially reflective element and aplurality of Petzval surfaces.

EC17) The method of EC1, wherein at least one camera of the respectivecamera-groups comprises a staggered array of electronic image sensors ata Petzval surface or a butted array of electronic image sensors at aPetzval surface.

EC18) The method of EC1, wherein at least one of the respectivecamera-groups comprises a plurality of cameras with parallel or nearlyparallel lenses, each camera comprising an array of electronic imagesensors at its Petzval surface(s), such that projected fields of view ofthe electronic image sensors overlap.

EC19) The method of EC1, wherein a Petzval surface for at least onecamera of the respective camera-groups comprises at least twoarea-format electronic image sensors or at least two line-formatelectronic image sensors.

EC20) The method of EC1, wherein the cardinal direction is a truecardinal direction or a magnetic cardinal direction.

EC21) The method of EC1, wherein the respective camera-groups comprise Nparticular camera-groups and the pre-determined angular offset isbetween 300/(2*N) and 420/(2*N) degrees.

EC22) The method of EC1, wherein the capturing oblique imagery comprisescapturing a plurality of images from at least a first one of therespective camera-groups.

EC23) The method of EC22, wherein the plurality of images are capturedsequentially in a strip.

EC24) The method of EC23, wherein the plurality of images comprisesfirst, second, and third contiguously obtained images, the second imageoverlaps by at least 50% with the first image, and overlaps by at least50% with the third image.

EC25) The method of EC23, wherein the plurality of images is a firstplurality of images, the strip is a first strip, the capturing obliqueimagery further comprises capturing a second plurality of images from atleast a second one of the respective camera-groups as a second strip,and the first strip and the second strip overlap with each other.

EC26) The method of EC25, wherein the first strip is captured at a firstperiod in time, the second strip is captured at a second period in time,and the first period in time is distinct from the second period in time.

EC27) The method of EC25, wherein a first image in the first stripoverlaps with a second image in the second strip, the first image iscaptured at a first period in time, the second image is captured at asecond period in time, and the first period in time is distinct from thesecond period in time.

EC28) The method of EC10, wherein the capturing oblique imagerycomprises capturing a first plurality of images from at least a firstone of the respective camera-groups and a second plurality of imagesfrom at least one camera of the nadir camera-group.

EC29) The method of EC28, wherein the first plurality of images iscaptured sequentially in a first image strip and the second plurality ofimages is captured sequentially in a second image strip.

EC30) The method of EC29, wherein the first image strip and the secondimage strip overlap.

EC31) The method of EC30, wherein a first image in the first image stripoverlaps with a second image in the second image strip, the first imageis captured at a first period in time, the second image is captured at asecond period in time, and the first period in time is distinct from thesecond period in time.

EC32) The method of EC1, wherein the capturing is performed by all ofthe respective camera-groups.

EC33) A method comprising operating a vehicle comprising one or morerespective camera-groups enabled to capture oblique imagery via adistortion correcting electronic image sensor.

EC34) The method of EC33, wherein the distortion correcting electronicimage sensor reduces angular separation between one of projected pixelrows and projected pixel columns of the distortion correcting electronicimage sensor and a pre-determined direction on the ground below apre-determined separation threshold.

EC35) The method of EC34, wherein the pre-determined direction on theground is a cardinal or intercardinal direction.

EC36) The method of EC34, wherein the pre-determined direction on theground is a nominal heading.

EC37) The method of EC34, wherein the pre-determined separationthreshold is 30 degrees.

EC38) The method of EC34, wherein the respective camera-groups areenabled to capture oblique imagery through a medium other than air.

EC39) The method of EC38, wherein the distortion correcting electronicimage sensor reduces distortions introduced at least in part by themedium, changes in the medium, or interfaces to the medium.

EC40) The method of EC38, wherein the medium is one or more of water,oil, and vacuum.

EC41) The method of EC33, wherein the distortion correcting electronicimage sensor comprises an electronic image sensor element with anon-zero twist angle.

EC42) The method of EC33, wherein the distortion correcting electronicimage sensor comprises a group of electronic image sensor elements andeach electronic image sensor element has an individual non-zero twistangle.

EC43) The method of EC33, wherein the distortion correcting electronicimage sensor comprises an electronic image sensor element with anon-uniform array of pixels.

EC44) The method of EC33, wherein the operating further comprisesconfiguring each of the respective camera-groups with a respectivepre-determined plan angle range.

EC45) The method of EC44, wherein at least one of the respectivepre-determined plan angle ranges includes an angle more than zerodegrees and less than 90 degrees.

EC46) The method of EC45, wherein the angle is approximately 45 degrees.

EC47) The method of EC45, wherein the operating is in accordance with anominal heading of the vehicle corresponding to a cardinal direction.

EC48) The method of EC45, wherein the operating is in accordance with anominal heading of the vehicle corresponding to an intercardinaldirection.

EC49) The method of EC33, wherein the operating further comprisesestablishing a nominal heading corresponding to a cardinal directionplus a pre-determined angular offset between 10 and 80 degrees, andcapturing oblique imagery with at least one camera of the respectivecamera-groups.

EC50) The method of EC33, wherein at least one of the respectivecamera-groups consists of a single camera.

EC51) The method of EC33, wherein at least one of the respectivecamera-groups comprises multiple cameras.

EC52) The method of EC33, wherein the respective camera-groups compriseN particular camera-groups, each of the N particular camera-groups isassociated with a unique integer K between 0 and (N−1) inclusive, and arespective pre-determined plan angle range of the particularcamera-group is (180+360*K)/N degrees plus a pre-determined angularoffset range.

EC53) The method of EC52, wherein the pre-determined angular offsetrange is between minus 120/N and plus 120/N degrees.

EC54) The method of EC53, wherein N is four or eight.

EC55) The method of EC33, wherein the vehicle further comprises at leastone nadir camera-group enabled to capture nadir imagery.

EC56) The method of EC55, wherein a sensor field of view within thenadir camera-group overlaps a sensor field of view within at least oneof the respective camera-groups.

EC57) The method of EC33, wherein the vehicle is one or more of anaircraft, an airplane, a lighter-than-air craft, a space-craft, ahelicopter, a satellite, a car, a truck, a land-based vehicle, a ship, aboat, a barge, a canoe, a submersible, and a submarine.

EC58) The method of EC57, wherein the vehicle is unmanned or manned

EC59) The method of EC33, wherein at least one electronic image sensorelement of the respective camera-groups is enabled to capture infraredradiation.

EC60) The method of EC33, wherein at least one of the respectivecamera-groups comprises an electronic image sensor element.

EC61) The method of EC33, wherein at least one camera of the respectivecamera-groups comprises at least one partially reflective element and aplurality of Petzval surfaces.

EC62) The method of EC33, wherein at least one camera of the respectivecamera-groups comprises a staggered array of electronic image sensorelements at a Petzval surface or a butted array of electronic imagesensor elements at a Petzval surface.

EC63) The method of EC33, wherein at least one of the respectivecamera-groups comprises a plurality of cameras with parallel or nearlyparallel lenses, each camera comprising an array of electronic imagesensor elements at its Petzval surface(s), such that projected fields ofview of the electronic image sensor elements overlap.

EC64) The method of EC33, wherein a Petzval surface for at least onecamera of the respective camera-groups comprises at least twoarea-format electronic image sensor elements or at least two line-formatelectronic image sensor elements.

EC65) The method of EC49, wherein the cardinal direction is a truecardinal direction or a magnetic cardinal direction.

EC66) The method of EC49, wherein the respective camera-groups compriseN particular camera-groups and the pre-determined angular offset isbetween 300/(2*N) and 420/(2*N) degrees.

EC67) The method of EC49, wherein the capturing oblique imagerycomprises capturing a plurality of images from at least a first one ofthe respective camera-groups.

EC68) The method of EC67, wherein the plurality of images are capturedsequentially in a strip.

EC69) The method of EC68, wherein the plurality of images comprisesfirst, second, and third images that are contiguously obtained, thesecond image overlaps by at least 50% with the first image, and overlapsby at least 50% with the third image.

EC70) The method of EC68, wherein the plurality of images is a firstplurality of images, the strip is a first strip, the capturing obliqueimagery further comprises capturing a second plurality of images from atleast a second one of the respective camera-groups as a second strip,and the first strip and the second strip overlap with each other.

EC71) The method of EC70, wherein the first strip is captured at a firstperiod in time, the second strip is captured at a second period in time,and the first period in time is distinct from the second period in time.

EC72) The method of EC70, wherein a first image in the first stripoverlaps with a second image in the second strip, the first image iscaptured at a first period in time, the second image is captured at asecond period in time, and the first period in time is distinct from thesecond period in time.

EC73) The method of EC55, wherein the vehicle further comprises at leastone nadir camera-group enabled to capture nadir imagery and thecapturing oblique imagery comprises capturing a first plurality ofimages from at least a first one of the respective camera-groups and asecond plurality of images from at least one camera of the nadircamera-group.

EC74) The method of EC73, wherein the first plurality of images iscaptured sequentially in a first image strip and the second plurality ofimages is captured sequentially in a second image strip.

EC75) The method of EC74, wherein the first and the second image stripsoverlap.

EC76) The method of EC75, wherein a first image in the first image stripoverlaps with a second image in the second strip, the first image iscaptured at a first period in time, the second image is captured at asecond period in time, and the first period in time is distinct from thesecond period in time.

EC77) The method of EC49, wherein the capturing is performed by all ofthe respective camera-groups.

EC78) A method comprising:

-   -   operating a vehicle comprising one or more respective        camera-groups enabled to capture oblique imagery via electronic        image sensor technology;    -   configuring each of the respective camera-groups with a        respective pre-determined plan angle range;    -   establishing a nominal heading as corresponding to a cardinal        direction plus a pre-determined angular offset between 10 and 80        degrees; and    -   capturing oblique imagery in some cardinal direction with at        least one camera of the respective camera-groups.

EC79) The method of EC78, wherein the respective camera-groups compriseN particular camera-groups, each of the N particular camera-groups isassociated with a unique integer K between 0 and (N−1) inclusive, andthe respective pre-determined plan angle range of the particularcamera-group is (180+360*K)/N degrees plus a pre-determined angularoffset range.

EC80) The method of EC78, further comprising configuring a particularelectronic image sensor of a particular one of the respectivecamera-groups in an orientation to reduce angular separation between thenominal heading and one of a projected pixel column and a projectedpixel row of the particular electronic image sensor below apre-determined separation threshold.

EC81) The method of EC80, wherein the configuring the particularelectronic image sensor comprises rotating the particular electronicimage sensor around an optical axis of a camera of the particularcamera-group.

EC82) The method of EC78, wherein the vehicle further comprises at leastone nadir camera-group enabled to capture nadir imagery.

EC83) The method of EC78, wherein the capturing oblique imagerycomprises capturing a plurality of images from at least a first one ofthe respective camera-groups.

EC84) The method of EC83, wherein the plurality of images are capturedsequentially in a strip.

EC85) The method of EC84, wherein the plurality of images comprisesfirst, second, and third contiguously obtained images, the second imageoverlaps by at least 50% with the first image, and overlaps by at least50% with the third image.

EC86) The method of EC84, wherein the plurality of images is a firstplurality of images, the strip is a first strip, the capturing obliqueimagery further comprises capturing a second plurality of images from atleast a second one of respective camera-groups as a second strip, andthe first strip and the second strip overlap with each other.

EC87) A method comprising:

-   -   operating a vehicle comprising one or more respective        camera-groups enabled to capture oblique imagery via a        distortion correcting electronic image sensor.

EC88) The method of EC87, wherein the distortion correcting electronicimage sensor reduces angular separation between one of projected pixelrows and projected pixel columns of the distortion correcting electronicimage sensor and a pre-determined direction on the ground below apre-determined separation threshold.

EC89) The method of EC88, wherein the pre-determined direction on theground is a nominal heading of the vehicle.

EC90) The method of EC87, wherein the distortion correcting electronicimage sensor comprises an electronic image sensor element with anon-zero twist angle.

EC91) The method of EC87, wherein the distortion correcting electronicimage sensor comprises a group of electronic image sensor elements andeach electronic image sensor element has an individual non-zero twistangle.

EC92) The method of EC87, wherein the operating further comprisesconfiguring each of the respective camera-groups with a respectivepre-determined plan angle range.

EC93) The method of EC87, wherein the operating further comprisesestablishing a nominal heading corresponding to a cardinal directionplus a pre-determined angular offset between 10 and 80 degrees, and theoperating further comprises capturing oblique imagery with at least onecamera of the respective camera-groups.

EC94) The method of EC93, wherein the capturing oblique imagerycomprises capturing a plurality of images from at least a first one ofthe respective camera-groups.

EC95) The method of EC94, wherein the plurality of images are capturedsequentially in a strip.

EC96) The method of EC95, wherein the plurality of images comprisesfirst, second, and third contiguously obtained images, the second imageoverlaps by at least 50% with the first image, and overlaps by at least50% with the third image.

EC97) The method of EC95, wherein the plurality of images is a firstplurality of images, the strip is a first strip of images, the capturingoblique imagery further comprises capturing a second plurality of imagesfrom at least a second one of respective camera-groups as a secondstrip, and the first strip and the second strip overlap with each other.

EC98) The method of EC97, wherein a first image in the first image stripoverlaps with a second image in the second strip and the first image iscaptured at a first period in time and the second image is captured at asecond period in time and the first period in time is distinct from thesecond period in time.

EC99) The method of EC87, wherein the respective camera-groups compriseN particular camera-groups, each of the N particular camera-groups isassociated with a unique integer K between 0 and (N−1) inclusive, and arespective pre-determined plan angle range of the particularcamera-group is (180+360*K)/N degrees plus a pre-determined angularoffset range.

EC100) The method of EC87, wherein the vehicle further comprises atleast one nadir camera-group enabled to capture nadir imagery.

EC101) A system comprising:

-   -   means for operating a vehicle comprising one or more respective        camera-groups enabled to capture oblique imagery via electronic        image sensor technology;    -   means for configuring each of the respective camera-groups with        a respective pre-determined plan angle range;    -   means for establishing a nominal heading of the vehicle as        corresponding to a cardinal direction plus a pre-determined        angular offset between 10 and 80 degrees; and    -   means for capturing oblique imagery in some cardinal direction        with at least one camera of the respective camera-groups.

EC102) The system of EC101, wherein the respective camera-groupscomprise N particular camera-groups, each of the N particularcamera-groups is associated with a unique integer K between 0 and (N−1)inclusive, and the respective pre-determined plan angle range of theparticular camera-group is (180+360*K)/N degrees plus a pre-determinedangular offset range.

EC103) The system of EC101, further comprising means for configuring aparticular electronic image sensor of a particular one of the respectivecamera-groups in an orientation to reduce angular separation between thenominal heading and one of a projected pixel column and a projectedpixel row of the particular electronic image sensor below apre-determined separation threshold.

EC104) The system of EC103, wherein the means for configuring comprisesmeans for rotating the particular electronic image sensor around anoptical axis of a camera of the particular camera-group.

EC105) The system of EC101, wherein the vehicle further comprises atleast one nadir camera-group enabled to capture nadir imagery.

EC106) The system of EC101, wherein the means for capturing obliqueimagery comprises means for capturing a plurality of images from atleast a first one of the respective camera-groups.

EC107) The system of EC106, wherein the plurality of images are capturedsequentially in a strip.

EC108) The system of EC107, wherein the plurality of images comprisesfirst, second, and third contiguously obtained images, the second imageoverlaps by at least 50% with the first image, and overlaps by at least50% with the third image.

EC109) The system of EC107, wherein the plurality of images is a firstplurality of image, the strip is a first strip, the means for capturingoblique imagery further comprises means for capturing a second pluralityof images from at least a second one of the respective camera-groups asa second strip, and the first strip and the second strip overlap witheach other.

EC110) A system comprising:

-   -   means for operating a vehicle comprising one or more respective        camera-groups enabled to capture oblique imagery via a        distortion correcting electronic image sensor.

EC111) The system of EC110, wherein the distortion correcting electronicimage sensor reduces angular separation between one of projected pixelrows and projected pixel columns of the distortion correcting electronicimage sensor and a pre-determined direction on the ground below apre-determined separation threshold.

EC112) The system of EC111, wherein the pre-determined direction on theground is a nominal heading.

EC113) The system of EC110, wherein the distortion correcting electronicimage sensor comprises an electronic image sensor element with anon-zero twist angle.

EC114) The system of EC110, wherein the distortion correcting electronicimage sensor comprises a group of electronic image sensor elements andeach electronic image sensor element has an individual non-zero twistangle.

EC115) The system of EC110, wherein the means for operating furthercomprises means for configuring each of the respective camera-groupswith a respective pre-determined plan angle range.

EC116) The system of EC110, wherein the means for operating furthercomprises means for establishing a nominal heading corresponding to acardinal direction plus a pre-determined angular offset between 10 and80 degrees, and the means for operating further comprise means forcapturing oblique imagery with at least one camera of the respectivecamera-groups.

EC117) The system of EC116, wherein the means for capturing obliqueimagery comprises means for capturing a plurality of images from atleast a first one of the respective camera-groups.

EC118) The system of EC117, wherein the plurality of images are capturedsequentially in a strip.

EC119) The system of EC118, wherein the plurality of images comprisesfirst, second, and third contiguously obtained images, the second imageoverlaps by at least 50% with the first image, and overlaps by at least50% with the third image.

EC120) The system of EC118, wherein the strip is a first strip, themeans for capturing oblique imagery further comprises means forcapturing a plurality of images from at least a second one of therespective camera-groups as a second strip, and the first strip and thesecond strip overlap with each other.

EC121) The system of EC120, wherein a first image in the first stripoverlaps with a second image in the second strip, the first image iscaptured at a first period in time, the second image is captured at asecond period in time, and the first period in time is distinct from thesecond period in time.

EC122) The system of EC110, wherein the respective camera-groupscomprise N particular camera-groups, each of the N particularcamera-groups is associated with a unique integer K between 0 and (N−1)inclusive, and a respective pre-determined plan angle range of theparticular camera-group is (180+360*K)/N degrees plus a pre-determinedangular offset range.

EC123) The system of EC110, wherein the vehicle further comprises atleast one nadir camera-group enabled to capture nadir imagery.

EC124) An apparatus comprising:

-   -   a vehicle comprising one or more respective camera-groups        enabled to capture oblique imagery via electronic image sensor        technology;    -   a camera mount assembly enabled to configure each of the        respective camera-groups with a respective pre-determined plan        angle range;    -   a navigation sub-system enabled to establish a nominal heading        of the vehicle as corresponding to a cardinal direction plus a        pre-determined angular offset between 10 and 80 degrees; and    -   an image capture sub-system enabled to capture oblique imagery        in some cardinal direction with at least one camera of the        respective camera-groups.

EC125) The apparatus of EC124, wherein the respective camera-groupscomprise N particular camera-groups, each of the N particularcamera-groups is associated with a unique integer K between 0 and (N−1)inclusive, and the respective pre-determined plan angle range of theparticular camera-group is (180+360*K)/N degrees plus a pre-determinedangular offset range.

EC126) The apparatus of EC124, further comprising a sensor mountassembly enabled to configure a particular electronic image sensor of aparticular one of the respective camera-groups in an orientation toreduce angular separation between the nominal heading and one of aprojected pixel column and a projected pixel row of the particularelectronic image sensor below a pre-determined separation threshold.

EC127) The apparatus of EC126, wherein the sensor mount assemblycomprises a sensor rotation assembly enabled to rotate the particularelectronic image sensor around an optical axis of a camera of theparticular camera-group.

EC128) The apparatus of EC124, wherein the vehicle further comprises atleast one nadir camera-group enabled to capture nadir imagery.

EC129) The apparatus of EC124, wherein the image capture sub-system isenabled to capture a plurality of images from at least a first one ofthe respective camera-groups.

EC130) The apparatus of EC129, wherein the image capture sub-systemcomprises an image strip capture sub-system enabled to capture theplurality of images sequentially in a strip.

EC131) The apparatus of EC130, wherein the plurality of images comprisesfirst, second, and third contiguously obtained images, the second imageoverlaps by at least 50% with the first image, and overlaps by at least50% with the third image.

EC132) The apparatus of EC130, wherein the plurality of images is afirst plurality of images, the strip is a first strip, the image stripcapture sub-system is further enabled to capture second a plurality ofimages from at least a second one of the respective camera-groups as asecond strip, and the first strip and the second strip overlap with eachother.

EC133) An apparatus comprising:

-   -   a vehicle comprising one or more respective camera-groups        enabled to capture oblique imagery via a distortion correcting        electronic image sensor.

EC134) The apparatus of EC133, wherein the distortion correctingelectronic image sensor reduces angular separation between one ofprojected pixel rows and projected pixel columns of the distortioncorrecting electronic image sensor and a pre-determined direction on theground below a pre-determined separation threshold.

EC135) The apparatus of EC134, wherein the pre-determined direction onthe ground is a nominal heading.

EC136) The apparatus of EC133, wherein the distortion correctingelectronic image sensor comprises an electronic image sensor elementwith a non-zero twist angle.

EC137) The apparatus of EC133, wherein the distortion correctingelectronic image sensor comprises a group of electronic image sensorelements and each electronic image sensor element has an individualnon-zero twist angle.

EC138) The apparatus of EC133, wherein the vehicle further comprises acamera mount assembly enabled to configure each of the respectivecamera-groups with a respective pre-determined plan angle range.

EC139) The apparatus of EC133, wherein the vehicle further comprises anavigation sub-system enabled to establish a nominal headingcorresponding to a cardinal direction plus a pre-determined angularoffset between 10 and 80 degrees, and the vehicle further comprises animage capture sub-system enabled to capture oblique imagery with atleast one camera of the respective camera-groups.

EC140) The apparatus of EC139, wherein the image capture sub-system isenabled to capture a plurality of images from at least a first one ofthe respective camera-groups.

EC141) The apparatus of EC140, wherein the image capture sub-systemcomprises an image strip capture sub-system enabled to capture theplurality of images sequentially in a strip.

EC142) The apparatus of EC141, wherein the plurality of images comprisesfirst, second, and third contiguously obtained images, the second imageoverlaps by at least 50% with the first image, and overlaps by at least50% with the third image.

EC143) The apparatus of EC141, wherein the plurality of images is afirst plurality of images, the strip is a first strip, the image stripcapture sub-system is further enabled to capture a second plurality ofimages from at least a second one of the respective camera-groups as asecond strip, and the first strip and the second strip overlap with eachother.

EC144) The apparatus of EC143, wherein a first image in the first stripoverlaps with a second image in the second strip, the first image iscaptured at a first period in time, the second image is captured at asecond period in time, and the first period in time is distinct from thesecond period in time.

EC145) The apparatus of EC133, wherein the respective camera-groupscomprise N particular camera-groups, each of the N particularcamera-groups is associated with a unique integer K between 0 and (N−1)inclusive, and a respective pre-determined plan angle range of theparticular camera-group is (180+360*K)/N degrees plus a pre-determinedangular offset range.

EC146) The apparatus of EC133, wherein the vehicle further comprises atleast one nadir camera-group enabled to capture nadir imagery.

EC147) A method comprising:

-   -   operating a vehicle comprising one or more respective        camera-groups each enabled to capture oblique imagery via        electronic image sensor technology;    -   configuring each of the respective camera-groups with a        respective pre-determined plan angle range that is any acute        angle modulo 90 degrees;    -   flying a flight plan comprising two or more flight line segments        over a collection area and capturing oblique imagery with at        least one camera of the respective camera-groups; and    -   wherein each of the respective camera-groups comprise at least        one electronic image sensor.

EC148) A method comprising:

-   -   operating a vehicle comprising one or more respective        camera-groups each enabled to capture oblique imagery via        electronic image sensor technology;    -   configuring each of the respective camera-groups with a        respective pre-determined plan angle range that is between 15        and 75 degrees modulo 90 degrees;    -   flying a flight plan comprising two or more flight line segments        over a collection area and capturing oblique imagery with at        least one camera of the respective camera-groups; and    -   wherein each of the respective camera-groups comprise at least        one electronic image sensor.

EC149) The method of EC147 or EC148, wherein a first one of therespective camera-groups is configured with a respective pre-determinedplan angle range that is between 15 and 75 degrees, and a second one ofthe respective camera-groups is configured with a respectivepre-determined plan angle range that is between 105 and 165 degrees.

EC150) The method of EC149, wherein a third one of the respectivecamera-groups is configured with a respective pre-determined plan anglerange that is between 195 and 255 degrees, and a fourth one of therespective camera-groups is configured with a respective pre-determinedplan angle range that is between 285 and 345 degrees.

EC151) The method of EC150, wherein at least one of the two or moreflight line segments is nominally parallel to a longest axis of thecollection area.

EC152) The method of EC151, wherein at least two of the two or moreflight line segments are nominally parallel to the longest axis.

EC153) The method of EC152, wherein at least the at least one cameracomprises one or more distortion correcting electronic image sensors.

EC154) The method of EC153, wherein at least one of the distortioncorrecting electronic image sensors is configured in accordance with atwist angle, and the twist angle is in accordance with any one or moreof a down angle of the at least one camera and position of the at leastone distortion correcting electronic image sensors within a field of alens of the at least one camera.

EC155) The method of EC154, wherein at least one of the respectivepre-determined plan angle ranges is pre-determined based at least inpart on a desired swath.

EC156) The method of EC155, wherein the flight plan is determined atleast in part programmatically based at least in part on the at leastone of the respective pre-determined plan angle ranges.

EC157) The method of EC156, wherein a 3D model of at least a portion ofthe collection area is formulated at least in part based on all or anyportions of image data collected via the at least one electronic imagesensor.

System and Operation

FIG. 5 conceptually illustrates a plan view of selected details of anembodiment of capturing oblique imagery via a camera-set with emergingoptical axes rotated in plan. For clarity of exposition, the cameras areconceptually identical to the one shown in FIGS. 2 and 3 (e.g., samealtitude, same down angle, focal length and image sensor size). Invarious embodiments, the nominal heading of vehicle 501 is anintercardinal direction (e.g., NW, NE, SW, SE). In some otherembodiments, the nominal heading of the vehicle is a cardinal direction(e.g., North, South, East, West). In some embodiments, the camera-setincludes four oblique cameras, with diagonal emerging optical axes 530,532, 534, 536. In various embodiments, the camera-set optionallyincludes an arbitrary number of cameras or camera-groups, e.g., two,three, four, or eight. The emerging optical axes of the cameras arerotated with respect to the nominal heading. In some embodiments, thereare four cameras with plan angles of approximately 45, 135, 225 and 315degrees. Note that if the nominal heading of the vehicle is anintercardinal direction and the cameras have plan angles ofapproximately 45, 135, 225 and 315 degrees, then the cameras captureoblique imagery from perspectives that are cardinal directionsSimilarly, if the nominal heading of the vehicle is a cardinal directionand the cameras have plan angles of approximately 45, 135, 225 and 315degrees, then the cameras capture oblique imagery from perspectives thatare intercardinal directions.

The projected field of view of each camera lens 504, 508, 512, 516 is anellipse that contains the respective projected sensor FOV 502, 506, 510,514, which is a trapezoid inscribed in the ellipse. The shape of thecamera lens' projected FOV and sensor FOV are due to the down and planangles of the cameras. An example sensor FOV has a long base 541, aright leg 542, a short base 543 and a left leg 544 and an exposure ofthe camera captures the interior of the sensor FOV. Additional capturedimages of the projected FOV trapezoids are shown, e.g., 520.

In some embodiments, adjacent strips of the ground are captured duringadjacent flight lines. To stitch these strips together, portions of thestrips are discarded (e.g., jagged edges) to ensure a smooth fit. Thenon-discarded portions are sometimes called the useful strip. The usefulstrip of ground captured by the camera corresponding to emerging opticalaxis 532 is between boundaries 552 and 554. The swath of the strip(e.g., width of the strip) is less than the shorter base of thetrapezoid, due to the spacing between each captured image. To dostereopsis on the captured images, each ground point is captured by twoconsecutive images. The swath of ground captured by two successiveimages is between boundaries 556 and 558. A wide swath with stereopsisoverlap in a rotated configuration uses cameras having a relatively highframe rate (e.g., frame spacing less than one fifth of the swath). Asthe frame rate gets higher and the stereopsis swath wider, thestereopsis baseline (length of camera translation between successiveimages) gets smaller, and thus the accuracy of depth perception bystereopsis gets worse.

For a rotated oblique camera the width of the swath (e.g., 555) isclosely related to the frame pitch (e.g., the distance between thecenters of successive frames along the nominal heading, which isdetermined by flight speed and image sensor frame rate), the down angle,and the difference between the plan angle (e.g., 532) and the nominalheading (e.g., 501). In some scenarios, a rotated oblique camera has aswath that is approximately 21% wider than the same oblique camera thatis parallel to the nominal heading. Note that the increase in swath isindependent of the nominal heading. The increase in swath from therotated oblique camera is potentially limited by the frame pitch. For arotated oblique camera, relatively smaller frame pitches result inrelatively larger increases in swath (relative to a Maltese-crossoblique camera); while relatively larger frame pitches result inrelatively smaller increases in swath and can potentially decrease thewidth of the swath.

The collection swath of a camera must fit within the projected FOVellipses. In FIG. 3, the forward and back swaths are constrained by theminor axis of the front and back FOV ellipses; the side-facing swathsare constrained by the major axis of the side-facing FOV ellipses, whichare significantly larger. In the example of FIG. 3, the sensor FOVs ofthe left and right cameras are 487 meters wide, and the sensor FOVs ofthe front and back cameras are 458 meters wide (distance 355).

In various embodiments, the swaths for all four cameras are equal, whichin some scenarios is an advantage compared to the camera configurationshown in FIG. 3. For example, the swath of the camera with emergingoptical axis 532 is bounded by inner edge 552 and outer edge 554. In anexample in the context of FIG. 5 with frame pitch of 100 meters, swath555 is 510 meters wide, which is approximately 11% wider than theminimum swath of an example in the context of FIG. 3. The FOVs forcameras on different sides of the vehicle are also spaced closertogether. In some embodiments, the larger swath enables the flight linesof the vehicle to be more broadly spaced, reducing the total number offlight lines and total distance traveled by the vehicle, which directlyreduces the cost of collecting the oblique imagery. In some embodiments,another advantage of more broadly spaced flight lines is that thevehicle speed during turns can be faster, so that less time is spentdecelerating and accelerating before and after turns.

FIG. 6 conceptually illustrates a plan view of selected details of anembodiment of capturing oblique imagery via a camera-set with diagonalemerging optical axes (e.g., plan angles of approximately 45, 135, 225and 315 degrees) and distortion correcting sensors. The cameras areconceptually identical to the camera illustrated in FIGS. 2, 3, and 5(e.g., same altitude, same down angle, focal length and image sensorsize, and same plan angles as in FIG. 5). However, the image sensors inthe cameras of FIG. 6 correct for the distortion caused by projectiononto the ground. The distortion correcting sensor in FIG. 6 is a twistedsensor. The image sensor is rotated around the optical axes of therespective cameras, so that the projected central pixel columns (orpixel rows) of the sensor are approximately aligned to a desireddirection on the ground (e.g., nominal heading of the vehicle or acardinal direction).

A second example of a distortion correcting sensor is a sensor with anon-uniform pixel array. The pixel array is distorted such that theprojected pixel columns (or pixel rows) of the sensor are approximatelyaligned to a desired direction on the ground (e.g., nominal heading ofthe vehicle or a cardinal direction).

In various embodiments, the nominal heading of the vehicle 601 is anintercardinal direction (e.g., NW, NE, SW, SE). In some otherembodiments, the nominal heading of the vehicle is a cardinal direction(e.g., North, South, East, West) or an arbitrary direction. Theprojected field of view of each camera lens 604, 608, 612, 616 is anellipse that contains the respective projected sensor FOVs 602, 606,610, 614, each a trapezium inscribed in the ellipse. The shape of thecamera's projected FOV and sensor FOV are due to the down and planangles of the cameras and the rotation of the sensor around the opticalaxis of the camera. An example sensor FOV has a long base 641, a rightleg 642, a short base 643 and a left leg 644 and an exposure of thecamera captures the interior of the sensor FOV. Additional capturedimages of the projected FOV trapeziums are shown, e.g., 620. Note thatif the nominal heading of the vehicle is an intercardinal direction andthe cameras have plan angles of approximately 45, 135, 225 and 315degrees, then the cameras capture oblique imagery from perspectives thatare cardinal directions. Similarly, if the nominal heading of thevehicle is a cardinal direction and the cameras have plan angles ofapproximately 45, 135, 225 and 315 degrees, then the cameras captureoblique imagery from perspectives that are intercardinal directions.

For an oblique camera with distortion correcting sensors, the width ofthe swath (e.g., 655) is closely related to the down angle, and thedifference between the plan angle (e.g., 632) and the nominal heading(e.g., 601). In some scenarios, a rotated oblique camera with distortioncorrecting sensors has a swath that is approximately 30% wider than thesame oblique camera that is parallel to the nominal heading. Note thatthe increase in swath is independent of the nominal heading. Note thatusing distortion correcting sensors in the oblique camera significantlyreduces or eliminates limitations related to frame pitch, compared tothe case of an oblique camera without distortion correcting sensors.

In various embodiments, the swaths for all four cameras are equal forany nominal heading. For example, the swath of the camera with emergingoptical axis 632 is bounded by inner edge 652 and outer edge 654. Thewidth of the swath is determined by the short base of the trapezium. Inthe example of FIG. 6, swath 655 is 593 meters wide, which isapproximately 30% wider than the minimum swath of the example from FIG.3. The FOVs for cameras on different sides of the vehicle are alsospaced closer together. For example, distance 665 between inner edge 662of the front-left swath and inner edge 652 of the front-right swath is898 meters, which is 43% closer together than the example from FIG. 3.In some embodiments, the larger swath enables the flight lines of thevehicle to be more broadly spaced, reducing the total number of flightlines and total distance traveled by the vehicle, which directly reducesthe cost of collecting the oblique imagery. In some embodiments, anadvantage of more broadly spaced flight lines is that the vehicle speedduring turns can be faster, so that less time is spent decelerating andaccelerating before and after turns.

Some embodiments have a different number and orientation of the camerasin the camera-set than the conceptual illustration in FIG. 6. Variousembodiments have fewer or more cameras (e.g., two, three, four, or eightcameras). Some embodiments have camera orientations that are asymmetricwith respect to the nominal heading (e.g., 5 cameras with plan angles of30, 60, 90, 120, and 150 degrees). In some embodiments, the camera-setincludes both cameras with distortion correcting sensors and cameraswithout distortion correcting sensors (e.g., 8 cameras, four withtwisted sensors and plan angles of 45, 135, 225, and 315 degrees, andfour with twist angles of zero and plan angles of zero, 90, 180, and 270degrees.).

In some embodiments, an advantage of rotated cameras with distortioncorrecting sensors is reducing the distance between the vehicle flightline projected to the ground and the inside edge of the oblique swath.As a result, in some embodiments the amount of extra area that istraveled around the edges of a collection area is reduced. When used forcollecting small areas (e.g., less than fifty square kilometers for theexample altitude, down angle, plan angle, and sensor size from FIG. 6),the reduced distance decreases the cost of collection by a relativelysmall amount. Additionally, for camera-sets where the nadir camera swathis intended to overlap the oblique swaths, more closely spaced obliqueswaths reduce the needed swath of the nadir camera, thereby making thenadir camera less expensive.

In various embodiments, an advantage of rotated cameras with distortioncorrecting sensors is reducing (e.g., reduced by approximately 35%) theprojected ground velocity on the Petzval surface, compared to theside-facing cameras of a Maltese Cross configuration. With a fixedexposure time, a lower projected ground velocity reduces the amount ofmotion blur and so improves visual quality.

In some embodiments, an advantage of rotated cameras with distortioncorrecting sensors is improved stereopsis. The swaths captured by theright and left forward rotated cameras are captured a few seconds laterby the respective rear rotated cameras, providing large-baseline,small-time-difference stereopsis for both sides of the vehicle. Incontrast, a Maltese Cross camera-set only captures a singlelarge-baseline, short-time-difference stereopsis between the forward,rear, and nadir cameras. Greater collection stereopsis enhances theprecision of the 3D ground points triangulated from the collectedimagery.

In various embodiments, the rotation of the Petzval surface and imagesensors cause the average projected pixel size to slightly increase insize, because the more remote portion of the FOV is better utilized.Equivalently, the average down angle of the pixels is slightly smaller.

In some embodiments, a rotated camera with distortion correcting sensorshas a wider swath than the equivalent camera in the forward or rearposition of a Maltese Cross (e.g., approximately 30% wider), but thesame number of cross-track (e.g., perpendicular to the nominal heading)pixels. So the average cross-track Ground Sample Distance (GSD) islarger (e.g., larger by approximately 30%). The average in-track (e.g.,parallel to the nominal heading) GSD is smaller (e.g., smaller by 30%),so that the average projected pixel area is only slightly larger (e.g.,larger by 5% or less). When the camera pixels are resampled into aNorth-East-West-South grid with uniform GSD north-south and east-west,the resolution differences between Maltese Cross and the rotated camerasis, in some scenarios, insignificant (e.g., less than 3% linearresolution). The rotated camera's smaller average in-track GSD leads tohigher pixel velocity at the Petzval surface (e.g., by about 30%).

FIG. 7 conceptually illustrates selected details of an example flightplan for an embodiment of capturing oblique imagery covering AlexandriaCounty, Virginia, using the camera-set of FIG. 6. Flight plan 701 isarranged in 25 flight lines (e.g., 702) with nominal headings northeastor southwest, separated by 24 turns (e.g., 703) and captures obliqueimages that are oriented north, south, east and west. FIG. 7 highlightsselected benefits, in some usage scenarios, of embodiments using arotated camera-set with twisted sensors. The total flight distance is193 kilometers, compared to 264 kilometers for a Maltese Cross systemand thus reduces the cost of collection by approximately 27%.

FIG. 8A conceptually illustrates selected details of the FOV of theforward camera from two adjacent flight lines for a Maltese Crosscamera-set capturing oblique imagery. In some scenarios there is someoverlap between the image strips swept out by these two swaths, but FIG.8A omits this overlap for clarity of presentation. Angle 802 is definedby the two camera positions on the two adjacent flight lines, and thepoint at which the two swaths join. When oblique imagery from the twoflight lines are stitched together, visual artifacts such as buildinglean will be less noticeable if angle 802 is smaller. Thus, minimizingand/or reducing angle 802 enables improved visual quality.

FIG. 8B conceptually illustrates selected details of the FOV of theforward camera from two adjacent flight lines for an embodiment ofcapturing oblique imagery via a camera-set with rotated emerging opticalaxes and distortion correcting sensors. In some scenarios there is someoverlap between the image strips swept out by these two swaths, but FIG.8B omits this overlap for clarity of presentation. Angle 804 between twoadjacent flight lines and the joint where the two swaths meet is smallerdue to the geometry of the camera-set and twisted sensors. When obliqueimagery from the two flight lines are stitched together, visualartifacts such as building lean are reduced, because angle 804 isrelatively smaller, resulting in superior visual quality.

FIG. 9 conceptually illustrates a plan view of selected details of anembodiment of capturing oblique and nadir imagery via a camera-set withrotated emerging optical axes and distortion correcting sensors, wherethe nadir and oblique swaths overlap slightly. The oblique cameras areconceptually identical to the one shown in FIG. 6 (e.g., same down andplan angles, focal length and image sensor size).

The projected field of view of the nadir camera lens 974 is a circlethat contains the projected sensor FOV 972, which is a square inscribedin the circle. The swath of the nadir camera is bounded by the ProjectedSensor FOV. The swath of the camera with emerging optical axis 932 isbounded by inner edge 952 and outer edge 954. Note that the swath of thenadir camera slightly overlaps the swath of the oblique camera, sincethe Projected Sensor FOV extends past the Inner Edge. However,simultaneous exposures on the nadir camera do not overlap with theoblique camera. The overlap enables relatively high quality imagery andcreates a triple baseline stereopsis for any given point in this range(e.g., two oblique shots and a nadir shot).

FIG. 10 conceptually illustrates a plan view of selected details ofembodiments of a vehicle traveling diagonally. Nominal Heading Limits1002 and 1003 form an angular offset range from a Cardinal Direction1011 (e.g., North). Vehicle 1000 establishes a Nominal Heading 1001 thatfalls between the Nominal Heading Limits (e.g., falling within theangular offset range). In some embodiments, the Nominal Heading isenabled to change as long as it stays within the Nominal Heading Limits(e.g., if the camera is mounted to the vehicle without a stabilizer).

In some embodiments of a Vehicle with a camera-set enabled to captureoblique imagery, the Nominal Heading Limits may be determined by thenumber of camera-groups in the camera-set. In some embodiments with Noblique camera-groups, the Nominal Heading Limits are 300/(2*N) and420/(2*N) degrees. For example, in an embodiment with 4 obliquecamera-groups, the angular offset range is 37.5-52.5 degrees(alternatively expressed as 45±7.5 degrees) from a cardinal direction,meaning that the vehicle travels diagonally, or approximately Northwest,Northeast, Southwest, or Southeast. In various scenarios, travelingdiagonally enhances the productivity of aerial image collection.

FIG. 11 conceptually illustrates a plan view of selected details ofembodiments of a vehicle with a rotated oblique camera-set. Vehicle 1000has a camera-set with any number of camera-groups enabled to captureoblique imagery (e.g., two, four, seven, eight, etc.), but for clarityonly a single camera-group is shown in FIG. 11. Plan Angle 1114 is theangle between Emerging Optical Axis 1111 and Nominal Heading 1101. TheEmerging Optical Axis Limits 1112 and 1113 form a plan angle range. Thecamera-group is configured such that the Emerging Optical Axis fallsbetween the Emerging Optical Axis Limits (e.g., falling within theangular separation range). This enables the Emerging Optical Axes to bebiased, as described in a subsequent section. Each camera-group has adifferent angular separation range and therefore a differentconfiguration. In various scenarios, the Emerging Optical Axis of acamera-group is allowed to vary during oblique image collection (e.g.,to accommodate a stabilizer), as long as the Emerging Optical Axis stayswithin the Emerging Optical Axis Limits.

In some embodiments with a rotated camera-set, the Emerging Optical AxisLimits of each camera-group are optionally determined by the number ofcamera-groups in the camera-set. In some embodiments with Ncamera-groups, the angular separation range of the Kth camera-group is(180+360*K)/N±120/N degrees from the Nominal Heading. For example, in anembodiment with 4 oblique camera-groups the angular separation rangesare 45±30, 135±30, 225±30, and 315±30 degrees from the Nominal Heading.If the established Nominal Heading of the Vehicle is a cardinaldirection (e.g., North), then the angular separation rangesapproximately correspond to Northwest, Northeast, Southwest, andSoutheast. If the established Nominal Heading of the Vehicle is anintercardinal direction (e.g., Northwest), then the angular separationranges approximately correspond to North, South, East, and West. Thisarrangement enables improved image quality and collection efficiency,such as when the camera-groups use distortion correcting electronicsensors. In other embodiments, a vehicle with a rotated camera-settravels diagonally while collecting oblique images, improving collectionefficiency and image quality.

Biased Emerging Optical Axes

In various embodiments, the emerging optical axes of the cameras in thecamera-set are statically biased towards the nominal heading. Forexample, with four cameras, the emerging optical axes are positioned at40, 140, 220, and 320 degrees from the nominal heading. The biasedconfiguration is, in some usage scenarios, beneficial because it reducesthe impact of the sun on image quality and thus extends the time windowfor collecting oblique imagery.

In some scenarios, the biased configuration biases the emerging opticalaxes away from the sun at various times of the day for certain travelpatterns (e.g., flying northeast-southwest in the northern hemispherebefore solar noon). In other usage models, the biasing reduces glarefrom the sun that is reflected off the ground (e.g., from water, dew,snow, etc.).

In other scenarios, the biasing reduces the distance between the nominalheading and the inside edges of the swaths of the oblique cameras. Thisdecreases the size of the nadir swath needed to have overlap between thenadir and oblique swathes, thereby decreasing the cost and complexity ofthe nadir camera.

Distortion Correcting Sensors

In various embodiments, the electronic image sensors in the cameras ofthe camera-set are distortion correcting electronic image sensors. Theemerging optical axis of an oblique camera is at an angle to the ground,herein called the down angle, for example between 20-70 degrees (oralternatively anywhere in the interval (0,90) degrees). As a result ofthe down angle, the sensor field of view is distorted when projectedthrough the camera lens to the ground. For example, a rectangular sensorprojects to a trapezium on the ground. In the case of a twist angleequal to zero, a rectangular sensor projects to a trapezoid on theground. In other scenarios, changes in the medium between the camera andthe ground conditionally distort the sensor FOV projection (e.g., if thecamera, mounted in air, is capturing an oblique view of the sea bottomthrough seawater under a horizontal glass window). An example of adistortion correcting sensor is a sensor that reduces this distortion,thereby improving sensor utilization and collection efficiency.

FIG. 12A conceptually illustrates selected details of embodiments of anoblique camera with an electronic image sensor that projects to adistorted sensor field of view. Electronic Image Sensor 1206 is arectangular, uniform array of pixels organized into rows and columns, anexample pixel being Pixel 1210. The Electronic Image Sensor is containedwithin Lens Field 1202, geometrically forming a rectangle inscribedwithin a circle. In the illustrated embodiment, the oblique camera ispart of a Maltese Cross camera-set. When projected to the ground, LensFOV 1204 is distorted vertically by the projection from a circle to anellipse. The Sensor FOV 1208 is similarly distorted from an inscribedrectangle to an inscribed trapezoid. Projected Pixel 1212 is the groundprojection of Pixel 1210 and demonstrates the transformation (e.g., acombined vertical and horizontal reflection) caused by the projection.

FIG. 12B conceptually illustrates selected details of embodiments of anoblique camera with a non-uniform distortion correcting electronic imagesensor that projects to a corrected sensor field of view. DistortionCorrecting Electronic Image Sensor 1226 is a trapezoidal, non-uniformarray of pixels organized into rows and columns, an example pixel beingPixel 1230. The Distortion Correcting Electronic Image Sensor iscontained within Lens Field 1222, geometrically forming a trapezoidinscribed within a circle. In the illustrated embodiment, the obliquecamera is part of a Maltese Cross camera-set. When projected to theground, the Lens FOV 1224 is distorted vertically by the projection froma circle to an ellipse. The Sensor FOV 1228 of the non-uniform pixelarray sensor is similarly distorted; however, it is distorted from aninscribed trapezoid to an approximate inscribed rectangle. Morespecifically, the non-uniform array of pixels is projected to a nearlyuniform array of pixels on the ground. Projected Pixel 1232 is theground projection of Pixel 1230 and demonstrates that in someembodiments, the non-uniform pixel array is designed to nearlycompletely cancel the distortion caused by the projection. Thisenhances, in some usage scenarios, the efficiency of oblique imagerycollection, as the entire swath of the camera is usable because thedistortion has been mostly corrected. Additionally, the projection ofthe pixels to the ground is relatively more uniform across the SensorFOV, which in some usage scenarios increases the minimum signal-to-noiseratio of the collected imagery across the entire swath, therebyincreasing the quality of the collected imagery. In various embodiments,the oblique camera is in a non-Maltese Cross configuration (e.g.,diagonal).

FIG. 13 conceptually illustrates selected details of embodiments of adiagonal oblique camera with a rotated distortion correcting electronicimage sensor that projects to a partially corrected sensor field ofview. Rotated Electronic Image Sensor 1306 is a rectangular, uniformarray of pixels organized into rows and columns, an example pixel beingPixel 1310. The Rotated Electronic Image Sensor is contained within LensField 1302, geometrically forming a rectangle inscribed within a circle.However, the Rotated Electronic Image Sensor is rotated around theoptical axis of the camera by Twist Angle 1314, which is the anglebetween Image Sensor Row Vector 1318 and Horizontal Vector 1316.

Because the oblique camera is projecting diagonally, the Lens FOV 1304is distorted vertically and horizontally by the projection from a circleto an ellipse. The Sensor FOV 1308 of the rotated sensor is similarlydistorted from a rotated inscribed rectangle to a rotated inscribedtrapezium. For example, Projected Pixel 1312 is a projection of Pixel1310 that is distorted. However, the distortion stretches the rotatedsensor FOV vertically and horizontally, thereby reducing the horizontaldistortion compared to an unrotated sensor. This enhances, in some usagescenarios, the efficiency of oblique imagery collection, as more of theswath of the camera is usable because the distortion has been reduced.Additionally, the projection of the pixels to the ground is relativelymore uniform across the Sensor FOV, which in some usage scenariosincreases the minimum signal-to-noise ratio of the collected imageryacross the entire swath, thereby increasing the quality of the collectedimagery. Conceptually, the non-uniform pixel array of FIG. 12B nearlycompletely corrects distortion while the Rotated Electronic Image Sensoris a linear approximation of a perfect correction.

In some embodiments, the twist angle of the electronic image sensor ispartially determined by the plan and down angles of the oblique camera.In various embodiments, for an oblique camera with plan and down anglesof 45 degrees, the twist angle is approximately 53 degrees. Thisconfiguration decreases the difference in length between the shortestand longest projected pixel row, improving collection efficiency. Insome embodiments, the twist angle is adjustable via an adjustmentmechanism. Example adjustment mechanisms include any one or more of ascrew, an actuator and a bearing (e.g., a flexure), and a piezoelectricactuator.

FIG. 14 conceptually illustrates selected details of embodiments of adiagonal oblique camera with a rotated array of rotated distortioncorrecting electronic image sensors that projects to an array ofpartially corrected sensor fields of view. The Rotated Array of RotatedElectronic Image Sensors is contained within Lens Field 1402,geometrically forming a rectangular array inside a circle. Conceptually,a first rotation applies to all electronic image sensors and anindividual rotation is also applied to each individual electronic imagesensor. Rotated Array of Rotated Electronic Image Sensors 1420 is arectangular array of multiple image sensors organized into rows. In someembodiments, the Rotated Array of Rotated Electronic Image Sensors is astaggered and/or butted array. In FIG. 14, the rows of electronic imagesensors are also organized into columns; in other embodiments, the rowsof electronic image sensors are staggered. Each electronic image sensoris a rectangular, uniform array of pixels organized into rows andcolumns, e.g., Rotated Electronic Image Sensor 1410. The entire RotatedArray of Rotated Electronic Image Sensors is rotated around the opticalaxis of the camera by Twist Angle 1414, which is the angle between ImageSensor Array Row Axis 1418 and Horizontal Vector 1416. In addition, eachrotated electronic image sensor is individually rotated around theoptical axis of the camera.

Because the oblique camera is projecting diagonally, the Lens FOV 1404is distorted vertically and horizontally by the projection from a circleto an ellipse. Projected Rotated Array of Rotated Electronic ImageSensors 1422 is similarly distorted from a rotated rectangular array toa rotated trapezium array. The sensor FOVs of the rotated sensors withinthe array (e.g., Sensor FOV 1412) are similarly distorted from rotatedrectangles to rotated trapeziums. However, the distortion stretches andshears the rotated array and the rotated sensor FOVs vertically andhorizontally, thereby improving the alignment of the e.g., columnvectors with the nominal heading compared to an unrotated array ofsensors. This enhances, in some usage scenarios, the efficiency ofoblique imagery collection because more of the swath of the camera isusable. Additionally, the projection of the pixels to the ground isrelatively more uniform across the Sensor FOVs, which in some usagescenarios increases the minimum signal-to-noise ratio of the collectedimagery across the entire swath, thereby increasing the quality of thecollected imagery. Conceptually, the non-uniform pixel array of FIG. 12Bnearly completely corrects distortion while the Rotated Array of RotatedElectronic Image Sensors is a piece-wise linear approximation of aperfect correction.

In some embodiments, the twist angle is determined by the plan and downangles, and the individual rotations are further determined by theposition of each electronic image sensor within the lens field. Anindividual rotation is determined with reference to a line bisecting thesensor FOV crossing the midpoints of the forward and rear edges of thesensor FOV. The individual rotation is varied until this bisected lineis aligned to a common axis (e.g., the nominal heading). In variousembodiments, for an oblique camera with 45 degree plan and down angles,the twist angle for the entire array is approximately 53 degrees and thetwist angles of the individual sensors relative to the array vary from−10 to +10 degrees. In various embodiments, the twist angle and/or theindividual rotations are adjustable via one or more adjustmentmechanisms. Example adjustment mechanisms include any one or more of ascrew, an actuator and a bearing, and a piezoelectric actuator.

Rotated electronic image sensors and rotated arrays of rotatedelectronic image sensors are usable with a variety of oblique cameras,camera-sets, vehicles and nominal headings. For example, one embodimentincludes a vehicle that travels on a nominal heading of approximately 45degrees from a cardinal direction with four oblique cameras configuredwith down angles of approximately 45 degrees, and plan angles ofapproximately 45, 135, 225 and 315 degrees, with the 45 and 225 degreeplan angle cameras including arrays of rotated image sensors with twistangles of 53 degrees, and with the 135 and 315 degree plan angle camerasincluding arrays of rotated image sensors with twist angles of −53degrees.

Oblique Imagery Collection and Analysis

FIG. 15 conceptually illustrates selected logical details of embodimentsof a vehicle-based image collection and analysis system. Note that inthe figure, for simplicity of representation, the various arrows areunidirectional, indicating direction of data flows in some embodiments.In various embodiments, any portions or all of the indicated data flowsare bidirectional and/or one or more control information flows arebidirectional. GIS system 1521 is a Geospatial Information System. Anexample of a GIS system is a computer running GIS software (e.g., ArcGISor Google Earth). In some embodiments, the GIS System plans the imagecollection process (e.g., selecting the flight path based on variousconditions and inputs). The GIS system is coupled to Logger Computer1522 wirelessly, e.g., via a cellular or WiFi network.

Vehicle 1520 includes an image collection platform, including one ormore Cameras 1501 . . . 1511, Logger Computer 1522, one or moreOrientation Sensors 1523, one or more Position Sensor 1524 elements,Storage 1525, and Autopilot 1528. Examples of a vehicle are a plane,e.g., a Cessna 206H, a Beechcraft B200 King Air, and a Cessna CitationCJ2. In some embodiments, vehicles other than a plane (e.g., a boat, acar, an unmanned aerial vehicle) include the image collection platform.

Cameras 1501 . . . 1511 include one or more image sensors and one ormore controllers, e.g., Camera 1501 includes Image Sensors 1502.1 . . .1502.N and controllers 1503.1 . . . 1503.N. In various embodiments, thecontrollers are implemented as any combination of any one or moreField-Programmable Gate Arrays (FPGAs), Application Specific IntegratedCircuits (ASICs), and software elements executing on one or more generaland/or special purpose processors. In some embodiments, each imagesensor is coupled to a controller, e.g., Image Sensor 1502.1 is coupledto Controller 1503.1. In other embodiments, multiple image sensors arecoupled to a single controller. Controllers 1503.1 . . . 1503.N . . .1513.1 . . . 1513.K are coupled to the Logger Computer, e.g., viaCameraLink, Ethernet, or PCI-Express and transmit image data to theLogger Computer. In various embodiments, one or more of the Cameras areenabled to capture oblique imagery. In some embodiments, one or more ofthe Cameras are enabled to capture nadir imagery.

The Orientation Sensors measure, record, and timestamp orientation data,e.g., the orientation of cameras. In various embodiments, theOrientation Sensors include one or more Inertial Measurement Units(IMUs), and/or one or more magnetic compasses. The Position Sensormeasures, records, and timestamps position data, e.g., the GPSco-ordinates of the Cameras. In various embodiments, the Position Sensorincludes one or more of a GPS sensor and/or linear accelerometers. TheOrientation Sensors and the Position Sensor are coupled to the LoggerComputer, e.g., via Ethernet cable and/or serial cable and respectivelytransmit timestamped orientation and position data to the LoggerComputer.

The Logger Computer is coupled to the Storage e.g., via PCI-Expressand/or Serial ATA, and is enabled to copy and/or move received data(e.g., from the Orientation Sensors, the Position Sensor, and/or theControllers) to the Storage. In various embodiments, the Logger Computeris a server and/or a PC enabled to execute logging software. The Storageincludes one or more forms of non-volatile storage, e.g., solid-statedisks and/or hard disks. In some embodiments, the Storage includes oneor more arrays, each array include 24 hard disks. In some embodiments,the Storage stores orientation, position, and image data.

The Autopilot is enabled to autonomously steer the Vehicle. In somescenarios, the Autopilot receives information that is manually enteredfrom the Logger Computer (e.g., read by the pilot via a display andtyped into the Autopilot).

Data Center 1526 includes one or more computers and further processesand analyzes image, position, and orientation data. In variousembodiments, the Data Center is coupled to the Storage via one or moreof wireless networking, PCI-Express, wired Ethernet, or othercommunications link, and the Storage further includes one or morecorresponding communications interfaces. In some embodiments, theStorage is enabled to at least at times communicate with the Data Centerover extended periods. In some embodiments, at least parts of theStorage at least at times perform short term communications buffering.In some embodiments, the Storage is enabled to at least at timescommunicate with the Data Center when the Vehicle is on the ground. Insome embodiments, one or more of the disks included in the Storage areremovable, and the disk contents are communicated to the Data Center viaphysical relocation of the one or more removable disks. The Data Centeris coupled to Customers 1527 via networking (e.g., the Internet) or byphysical transportation (e.g., of computer readable media).

FIG. 16 illustrates a flow diagram of selected details of an embodimentof image collection and analysis wherein the vehicle is a plane. Invarious embodiments, a collection area is selected (e.g., from acustomer or an operator of the aerial image collection and analysissystem). An example of a collection area is a defined geographic region,e.g., a state, a county, or a set of latitude and longitude boundaries.The collection area is programmed into a GIS system in action 1601.

Based on requirements such as desired resolution, ground elevation inthe collection area, weather patterns, desired collection overlap, andother factors, the GIS system determines flight altitude and diagonalflight line pitch in action 1602. The flight line pitch is determined inaccordance with any increased swath enabled by rotated camera-groupsoptionally with distortion correcting electronic image sensors. Forexample, the flight altitude and the diagonal line pitch are selected toachieve the desired resolution (e.g., 10 cm GSD) and ensure that theswaths corresponding to the diagonal flight lines overlap sufficiently(e.g., 5%), accounting for variation in swath width from variations inthe altitude above ground (e.g., caused by mountains). In variousembodiments, the resolution of the collected imagery is increased byflying the vehicle lower to the ground, while the area collected isincreased by flying the vehicle higher above the ground. In someembodiments, the altitude may be determined to fly below clouds or otherweather that would interfere with image collection.

Once the flight altitude and the diagonal line pitch are known, the GISsystem converts the collection area into a list of diagonal linesegments (e.g., line segments that run NW, NE, SE, SW) in action 1603,based on the flight altitude and the diagonal line pitch. If flown, thediagonal line segments cover the collection region.

In action 1604, the GIS system creates a diagonal flight plan byselecting multiple diagonal line segments and connecting them into asingle path. In some usage scenarios and/or embodiments, the flight planis designed to minimize fuel consumption and connects adjacent diagonalline segments with a single turn. For example, FIG. 7 illustrates ahypothetical flight plan covering Alexandria County, Virginia.

The flight plan is transmitted from the GIS system to the loggercomputer (e.g., via a cellphone network), where it is read by the pilot(e.g., via a display or a tablet computer) and manually entered into theautopilot. Once the flight plan is entered, the pilot flies the flightplan, with the autopilot controlling the vehicle throughout most of theflight plan, and the image collection platform collects the image,position, and orientation data in action 1605. The image sensor capturesimage data, that is then compressed and timestamped by the controllers.In some embodiments, the image sensor timestamps image data. In variousembodiments, the captured image data is one or more of oblique imagedata and nadir image data. The logger computer receives timestampedorientation data from the orientation sensors, timestamped position datafrom the GPS sensors, and timestamped image data from the controllers,and writes the timestamped orientation, position, and image data to thestorage. In some embodiments, the timestamped orientation data isdiscarded and is not written to the storage. In some scenarios,collecting the image, position, and orientation data takes many hours.

In some scenarios, a problem occurs in one or more diagonal flight linesegments (e.g., data is incorrectly captured or written incorrectly tostorage). In various embodiments, the pilot conditionally directs theautopilot to fly these diagonal flight line segments again, to recollecttimestamped image, orientation, and position data, either during thesame flight or as part of a subsequent flight. In some embodiments, thevehicle is unmanned and the flight plan is programmed before flight orduring flight via remote control.

In various embodiments, any one or more of the line pitch, the linesegments, and the flight plan are other than diagonal, such as acardinal direction (e.g. north, south, east, and west), an intercardinaldirection (e.g. northeast, northwest, southeast, and southwest), adirection determined to be oriented parallel to a longest axis of acollection area, or any particular direction. In various embodiments,the GIS system determines any one or more of the line pitch, the linesegments, and the flight plan in accordance with one or more respectiveplan angles associated with one or more camera-groups, independent oforientation of the flight plan.

In various embodiments, any one or more twist angles are specified(optionally in conjunction with one or more respective plan angles) tothe GIS system to enable the GIS system to determine an optimal or morenearly optimal flight plan. In various embodiments, one or more twistangles and/or plan angles are specified by the GIS system as ancillarydata to a flight plan to form, in aggregate, an enhanced flight plan. Invarious embodiments, one or more twist angles and/or plan angles areprogrammable, such that the twist angle and/or plan angle is configuredautomatically when an enhanced flight plan is loaded.

When the collection is finished, the vehicle stops (e.g., via landing)and the timestamped image, orientation, and position data is moved fromthe storage to the data center. In some embodiments, the data is movedor copied from the storage to the data center (e.g., over a network, orvia PCI-Express). In other embodiments, the storage is physically movedfrom the vehicle into the data center. In some embodiments, the image,orientation, and position data is further processed in the data centerin action 1606. In some embodiments, the image, orientation, andposition data is processed; and strips of sequentially captured andoverlapping (e.g., by 60%) images are stitched together to form a 2Dmosaic of the image collection area (e.g., one mosaic corresponding toeach camera). In some embodiments, triangulation is used to produce a 3Dmodel of the collection area from the collected image, orientation, andposition data (e.g. from two or more cameras). In some scenarios, theprocessed imagery is optionally resampled to a different resolution(e.g., data is collected with 10 cm GSD, and downsampled to 20 cm GSD;alternatively, data is collected at 10 cm GSD and super-resolved to 20cm GSD). In some embodiments, the processed imagery is further analyzedto identify specific features, e.g., a damaged house, a damaged roof, abody, or a tree in proximity to a structure.

Once the imagery has been processed and/or analyzed, all or any portionsof results of the processing and/or analyzing is sent to customers asdata in action 1607, via the Internet or physical transport of computerreadable media (e.g., a hard disk and/or a DVD-ROM, or any othernon-volatile storage media). In various embodiments, the datatransmitted to the customer is processed imagery, e.g., processedimagery of the collection area. In some embodiments, analyzed imagery issent to the customer, e.g., the number of houses in the collection area.

In some embodiments, when the flight line segments are diagonal, theprocessed 2D mosaics have perspectives in cardinal directions. In someembodiments, the flight line segments are in arbitrary directions, whilestill increasing the swath of the collection, but the perspective of the2D mosaics is in non-cardinal directions. In some embodiments, theflight line segments and the flight plan are vehicle travel lines andvehicle travel plans e.g., for a car or boat traveling across acollection area.

FIG. 17 conceptually illustrates selected physical details ofembodiments of a vehicle-based image collection and analysis system.

Vehicle 1701 includes the image collection platform, including one ormore cameras (e.g., Camera 1705), Logger Computer 1703, Display 1704,one or more Orientation and Position Sensors 1710, Storage 1702, andAutopilot 1711. Examples of the Vehicle include a plane, e.g., a Cessna206H, a Beechcraft B200 King Air, and a Cessna Citation CJ2. In someembodiments, vehicles other than a plane (e.g., a boat, a car, anunmanned aerial vehicle) include the image collection platform.

The Cameras include one or more image sensors and one or morecontrollers, e.g., Camera 1705 includes Image Sensors 1707 andControllers 1706. Each of the cameras is pointed towards the ground atan oblique angle, through a view port. In some embodiments, the viewport is climate controlled to reduce condensation and temperaturegradients to improve the quality of captured image data. In variousembodiments, the cameras are stabilized to reduce vibration and shockfrom the vehicle (e.g., vibrations from the engine, shock fromturbulence), thereby improving the quality of captured image data. Invarious embodiments, storage is removable from the vehicle for physicaltransport to a data center.

In various embodiments, any one or more of Camera 1705 and Camera 1501are embodiments and/or implementations of one another. In variousembodiments, any one or more of Image Sensor 1707, Image Sensors 1502.1. . . 1502.N . . . 1512.K, Distortion Correcting Electronic Image Sensor1226, Rotated Electronic Image Sensor 1306, and Rotated Electronic ImageSensor 1410 are embodiments and/or implementations of one another.

Example Implementation Techniques

In various embodiments the vehicle is an airplane, helicopter,lighter-than-air craft, boat, ship, barge, submersible, satellite,space-craft, car, or truck. In various embodiments, the vehicles arevariously manned or unmanned.

In some embodiments, rather than having a single electronic image sensorbehind each camera lens, a mosaic of several sensors is used. The mosaicis assembled at a single Petzval surface at the rear of the lens. Inother embodiments, the lens admits light through a series of partiallyreflecting surfaces, so that the image sensors are assembled ontomultiple surfaces, with the active areas overlapping. In variousembodiments, the partially reflecting surfaces are spectrally selective,to use the different sensors to capture different portions of theelectromagnetic spectrum. In some embodiments, the partially reflectivesurfaces are polarization selective, to use the different sensors tocapture the polarization information of the incoming light. In yet otherembodiments, the reflecting surfaces divide the incoming light evenlybetween multiple Petzval surfaces. In various embodiments, the mosaicincludes several line-format sensors, each collecting light fromdifferent portions of the spectrum.

In some embodiments, a mosaic of line-format sensors is used at theforward and rear edges of the field of view of the lenses, so that thesame points on the ground are collected from view angles approximately,e.g., 10 degrees apart, at times separated by, e.g., a few seconds. Tocapture a combination of depth and spectral information, each lenscarries behind it a mosaic of both line-format and area-format sensors.The resulting images are useful for extracting 3D depth information froma scene.

In various embodiments, a vehicle collects oblique imagery (andoptionally nadir imagery) along a nominal heading using a plurality ofcamera-groups. For a first example, two camera-groups are oriented at asame down angle, optionally with a nadir camera. Each of thecamera-groups is oriented at a respective plan angle, such as theta and180 degrees minus theta, or alternatively 180 degrees plus theta and 360degrees minus theta. For a second example, four camera-groups areoriented at a same down angle, optionally with a nadir camera. Each ofthe camera-groups is oriented at a respective plan angle, such as theta,180 degrees minus theta, 180 degrees plus theta, and 360 degrees minustheta.

In the first and the second examples, the same down angle is variouslybetween 20-70 degrees (or alternatively anywhere in the interval (0,90)degrees), according to embodiment and/or usage scenario. In the firstand the second examples, theta is any value, such as between 35 and 55degrees, with specific exemplary values being 40, 45, or 50 degrees,according to embodiment and/or usage scenario. In the first and thesecond examples, the nominal heading is any value, such as a cardinaldirection (e.g. north, south, east, and west), an intercardinaldirection (e.g. northeast, northwest, southeast, and southwest), or adirection determined to be oriented parallel to a longest axis of acollection area.

At least one of the camera-groups includes one or more electronic imagesensors. In some embodiments, the orienting of camera-groups at a downangle (e.g. to obtain oblique imagery) introduces distortion to imagesformed on the electronic image sensors.

In some embodiments, any one or more of the electronic image sensors arenot enabled to correct the distortion, and in other embodiments, any oneor more of the electronic image sensors are enabled to wholly orpartially correct for the distortion. Some of the non-distortioncorrecting image sensors have a zero twist angle. Some of the distortioncorrecting image sensors have a non-zero twist angle, e.g., to alignprojected rows (or alternatively columns) of the sensors in a particularmanner, such as aligned to the nominal heading, a cardinal direction, anintercardinal direction, or any other alignment. Some of the distortioncorrecting image sensors include a plurality of sensor elementsassociated with a particular camera of one of the camera-groups. Theplurality of sensors elements is collectively rotated (e.g. by anon-zero twist angle) around an optical axis of the camera, and each ofthe sensor elements is individually rotated around the optical axis.Some of the non-distortion correcting image sensors have uniform pixelarrays. Some of the distortion correcting image sensors have non-uniformpixel arrays. Some of the distortion correcting image sensors enableimage collection with a wider swath than an otherwise identical contextwith non-distortion correcting image sensors.

CONCLUSION

Certain choices have been made in the description merely for conveniencein preparing the text and drawings and unless there is an indication tothe contrary the choices should not be construed per se as conveyingadditional information regarding structure or operation of theembodiments described. Examples of the choices include: the particularorganization or assignment of the designations used for the figurenumbering and the particular organization or assignment of the elementidentifiers (the callouts or numerical designators, e.g.) used toidentify and reference the features and elements of the embodiments.

The words “includes” or “including” are specifically intended to beconstrued as abstractions describing logical sets of open-ended scopeand are not meant to convey physical containment unless explicitlyfollowed by the word “within.”

Although the foregoing embodiments have been described in some detailfor purposes of clarity of description and understanding, the inventionis not limited to the details provided. There are many embodiments ofthe invention. The disclosed embodiments are exemplary and notrestrictive.

It will be understood that many variations in construction, arrangement,and use are possible consistent with the description, and are within thescope of the claims of the issued patent. The order and arrangement offlowchart and flow diagram process, action, and function elements arevariable according to various embodiments. Also, unless specificallystated to the contrary, value ranges specified, maximum and minimumvalues used, or other particular specifications (such as number andconfiguration of cameras or camera-groups, number and configuration ofelectronic image sensors, nominal heading, down angle, twist angles,and/or plan angles), are merely those of the described embodiments, areexpected to track improvements and changes in implementation technology,and should not be construed as limitations.

Functionally equivalent techniques known in the art are employableinstead of those described to implement various components, sub-systems,operations, functions, routines, sub-routines, in-line routines,procedures, macros, or portions thereof.

The embodiments have been described with detail and environmentalcontext well beyond that required for a minimal implementation of manyaspects of the embodiments described. Those of ordinary skill in the artwill recognize that some embodiments omit disclosed components orfeatures without altering the basic cooperation among the remainingelements. It is thus understood that much of the details disclosed arenot required to implement various aspects of the embodiments described.To the extent that the remaining elements are distinguishable from theprior art, components and features that are omitted are not limiting onthe concepts described herein.

All such variations in design are insubstantial changes over theteachings conveyed by the described embodiments. It is also understoodthat the embodiments described herein have broad applicability to otherimaging, survey, surveillance, and photogrammetry applications, and arenot limited to the particular application or industry of the describedembodiments. The invention is thus to be construed as including allpossible modifications and variations encompassed within the scope ofthe claims of the issued patent.

1. (canceled)
 2. A method comprising: operating a vehicle in a nominalheading; capturing oblique imagery of a surface via one or morerespective camera-groups comprising 4 particular camera-groups havingrespective particular plan angles of approximately 45, 135, 225, and 315degrees with respect to the nominal heading, at least one of the 4particular camera-groups comprising at least one camera comprising atleast one distortion correcting electronic image sensor; and wherein theat least one distortion correcting electronic image sensor comprises oneor more one-dimensional collections of a plurality of pixel elements,and at least a particular one of the one-dimensional collections isoriented to align a projection of the particular one-dimensionalcollection with the nominal heading.
 3. The method of claim 2, whereinthe one-dimensional collections correspond to one of a collection ofrows and a collection of columns of the at least one distortioncorrecting electronic image sensor.
 4. The method of claim 2, whereinthe capturing oblique imagery is in accordance with a down angle of theat least one camera, and the orienting is based at least in part on thedown angle.
 5. The method of claim 2, wherein the orienting is inaccordance with any one or more of increasing a swath width andincreasing uniformity of projection of pixels onto the surface.
 6. Themethod of claim 2, further comprising: wherein the capturing obliqueimagery comprises capturing a plurality of images from at least a firstone of the respective camera-groups; wherein the plurality of images iscaptured sequentially in a strip; wherein the plurality of images is afirst plurality of images, the strip is a first strip, the capturingoblique imagery further comprises capturing a second plurality of imagesfrom at least a second one of the respective camera-groups as a secondstrip, and the first strip and the second strip overlap with each other;and wherein a first image in the first strip overlaps with a secondimage in the second strip, the first image is captured at a first periodin time, the second image is captured at a second period in time, andthe first period in time is distinct from the second period in time. 7.The method of claim 2, wherein the vehicle is a flying vehicle and thesurface is the ground.
 8. The method of claim 2, wherein the nominalheading is nominally an intercardinal direction.
 9. A method comprising:operating a vehicle in a nominal heading; capturing oblique imagery of asurface via one or more respective camera-groups comprising 4 particularcamera-groups having respective particular plan angles of approximately45, 135, 225, and 315 degrees with respect to the nominal heading, atleast one camera of at least one of the 4 particular camera-groupscomprising at least one distortion correcting electronic image sensor;and wherein the at least one camera has an associated Petzval surface,and the at least one distortion correcting electronic image sensor isrotated at an angle based at least in part on a position of the at leastone distortion correcting electronic image sensor in the Petzvalsurface.
 10. The method of claim 9, further comprising: wherein thecapturing oblique imagery comprises capturing a plurality of images fromat least a first one of the respective camera-groups; wherein theplurality of images is captured sequentially in a strip; wherein theplurality of images is a first plurality of images, the strip is a firststrip, the capturing oblique imagery further comprises capturing asecond plurality of images from at least a second one of the respectivecamera-groups as a second strip, and the first strip and the secondstrip overlap with each other; and wherein a first image in the firststrip overlaps with a second image in the second strip, the first imageis captured at a first period in time, the second image is captured at asecond period in time, and the first period in time is distinct from thesecond period in time.
 11. The method of claim 9, wherein the vehicle isa flying vehicle and the surface is the ground.
 12. The method of claim9, wherein the nominal heading is nominally an intercardinal direction.13. An apparatus comprising: one or more respective camera-groupscomprising 4 particular camera-groups enabled to capture oblique imageryof a surface, the respective camera-groups enabled to operate in avehicle in accordance with a nominal heading, the 4 particularcamera-groups having respective particular plan angles of approximately45, 135, 225, and 315 degrees with respect to the nominal heading, atleast one camera of at least one of the 4 particular camera-groupscomprising at least one distortion correcting electronic image sensor;and wherein the at least one distortion correcting electronic imagesensor comprises one or more one-dimensional collections of a pluralityof pixel elements, and at least a particular one of the one-dimensionalcollections is oriented to align a projection of the particularone-dimensional collection with the nominal heading.
 14. The apparatusof claim 13, wherein each of the one-dimensional collections correspondto one of respective rows and respective columns of the at least onedistortion correcting electronic image sensor.
 15. The apparatus ofclaim 13, wherein the capturing oblique imagery is in accordance with adown angle of the at least one camera, and the orienting is based atleast in part on the down angle.
 16. The apparatus of claim 13, whereinthe orienting is in accordance with any one or more of increasing aswath width and increasing uniformity of projection of pixels onto thesurface.
 17. The apparatus of claim 13, further comprising: wherein thecapturing oblique imagery comprises capturing a plurality of images fromat least a first one of the respective camera-groups; wherein theplurality of images is captured sequentially in a strip; wherein theplurality of images is a first plurality of images, the strip is a firststrip, the capturing oblique imagery further comprises capturing asecond plurality of images from at least a second one of the respectivecamera-groups as a second strip, and the first strip and the secondstrip overlap with each other; and wherein a first image in the firststrip overlaps with a second image in the second strip, the first imageis captured at a first period in time, the second image is captured at asecond period in time, and the first period in time is distinct from thesecond period in time.
 18. The apparatus of claim 13, wherein thevehicle is a flying vehicle and the surface is the ground.
 19. Anapparatus comprising: one or more respective camera-groups comprising 4particular camera-groups enabled to capture oblique imagery of asurface, the respective camera-groups enabled to operate in a vehicle inaccordance with a nominal heading, the 4 particular camera-groups havingrespective particular plan angles of approximately 45, 135, 225, and 315degrees with respect to the nominal heading, at least one camera of atleast one of the 4 particular camera-groups comprising at least onedistortion correcting electronic image sensor; and wherein the at leastone camera has an associated Petzval surface, and the at least onedistortion correcting electronic image sensor is rotated at an anglebased at least in part on a position of the at least one distortioncorrecting electronic image sensor in the Petzval surface.
 20. Theapparatus of claim 19, further comprising: wherein the capturing obliqueimagery comprises capturing a plurality of images from at least a firstone of the respective camera-groups; wherein the plurality of images iscaptured sequentially in a strip; wherein the plurality of images is afirst plurality of images, the strip is a first strip, the capturingoblique imagery further comprises capturing a second plurality of imagesfrom at least a second one of the respective camera-groups as a secondstrip, and the first strip and the second strip overlap with each other;and wherein a first image in the first strip overlaps with a secondimage in the second strip, the first image is captured at a first periodin time, the second image is captured at a second period in time, andthe first period in time is distinct from the second period in time. 21.The apparatus of claim 19, wherein the vehicle is a flying vehicle andthe surface is the ground.